torque ripple reduction in direct torque control induction motor drive ...

TORQUE RIPPLE REDUCTION IN DIRECT TORQUE CONTROL INDUCTION MOTOR DRIVE USING SVM AND FLDRC Obbu CHANDRA SEKHAR*, Dr. Koritala CHANDRA SEKHAR** * Vignan`s Lara Institute of Technology and Science, Vadlamudi, Guntur, India ** R.V.R. & J.C. College of Engineering, Chowdavaram, Guntur, India Abstract. Direct Torque Control (DTC) is a control technique used in AC drive systems to obtain high performance torque control. The conventional DTC drive contains a pair of hysteresis comparators, a flux and torque estimator and a voltage vector selection table. The torque and flux are controlled simultaneously by applying suitable voltage vectors, and by limiting these quantities within their hysteresis bands, de-coupled control of torque and flux can be achieved. However, DTC drives utilizing hysteresis comparators suffer from high torque ripple and variable switching frequency. This work aims at developing a DTC scheme for induction motor, with reduced toque ripple, using fuzzy logic duty ratio control (FLDRC) and Space Vector Modulation (SVM) techniques. From the simulation results shows that feeding electrical drive with proposed system greatly improves the drive performance. The performance of this control method has been demonstrated by simulations performed using a versatile simulation package, MATLAB/SIMULINK. Keywords: DTC, Fuzzy logic, Duty ratio control, SVM, Induction motor, MATLAB/SIMULINK.

period and the zero switching state is applied for the rest of the switching period and with this the ripple is reduced considerably as compared to conventional DTC [8 - 13]. However, the basis of the SVM-DTC methodology is the calculation of the required voltage space vector to compensate the flux and torque errors exactly by using a predictive technique and then its generation using the SVM at each sample period. In this paper, the simulations of different DTC schemes (Conventional DTC, DTC using duty ratio control technique and DTC using SVM) were carried out using MATLAB/SIMULINK simulation package and finally the results are compared based on their simulation results.

1. Introduction The induction motor due to its well known advantages of simple construction, reliability, ruggedness, and low cost has found very wide industrial applications. These advantages are superseded by control problems when using an IM in industrial drives with high performance demands. Using DTC or direct self control (DSC) it is possible to obtain a good dynamic control of the torque without any mechanical transducer on the machine shaft. Thus DTC and DSC can be considered as "sensor less type" control techniques [1 - 6]. DSC is preferable in the high power range applications where a lower inverter switching frequency can justify higher current distortion. DTC is more suitable in the small and medium power range application .The basic concept of direct torque control induction motor drives is to control both stator flux and electromagnetic torque of the machine simultaneously. The direct torque control (DTC) based drives do not require the coordinate transformation between stationary frame and synchronous frame in comparison with the conventional vector controlled drives [7]. The name direct torque control (DTC) is derived by the fact that on the basis of the error between the reference and the estimated values of the torque and flux, it is possible to directly control the inverter states in order to reduce the torque and flux error within limits. In duty ratio control technique, the voltage vector is not applied for the entire switching period; instead it is only applied for a portion of switching

2. DTC Scheme Figure 1 shows a simple structure of the conventional DTC for Induction motor drive. In DTC the reference to be applied is directly calculated from the equation of the load, usually an Induction Motor (IM). In the following, a short description of DTC is presented, just to introduce to its extension to multilevel VSI. Considering Park transform of IM equations, it is possible to write in

r

r

equation (1), where ϕS is the stator flux, U S , iS and

rS are the stator voltage, current and resistance respectively.

dϕ S = U S − rS ⋅ iS dt

(1)

Ignoring the contribution of the current, which 5

RECENT, Vol. 14, no. 1(37), March, 2013 technique is used to obtain the voltage space vector required to exactly compensate the flux and torque errors. The torque ripple for this SVMDTC is significantly improved and switching frequency is maintained constant. SVM is based on the switching between two adjacent boundary active vectors and a zero space vector. Space vector modulation (SVM) is one of the preferred real-time modulation techniques and is widely used for digital control of voltage source inverters.

can be considered small in the respect of the stator voltage, the variation of stator flux can be ascribed all to the voltage applied. So, a proportional relationship between flux variation and voltage in a given cycle Tc can be found by discretizing (1).

∆ϕS ≅ Tc ⋅ iS

(2)

Figure 2. DTC Principles: vector representation of the stator and rotor fluxes during a sample interval TC

A typical space vector diagram for the twolevel inverter is shown in Figure 3, where the six active vectors V1 to V6 form a regular hexagon with six equal sectors (I to VI). The zero vector V0 lies on the center of the hexagon.

Figure 1. The conventional DTC scheme of IM drive system

Analyzing the equation binding the stator and rotor fluxes (ϕS and ϕr) to the torque (Te), it is possible to find that an augmentation of the angle between fluxes ( ϑ sr) means an augmentation of torque, as (3) shows, where M, σ, LS and p are the mutual inductance, the leakage inductance and number of poles respectively.

Te =

3 p M ⋅ ⋅ ⋅ ϕ S ⋅ ϕr ⋅ sin ϑ sr 2 2 σ ⋅ LS

(3)

The relationship between stator and rotor fluxes it can be assumed that a fast variation of the stator flux angular speed will reflect in an increment of the angle ϑ sr as Figure 2 schematically shows. So, imposing a particular stator voltage, it is possible to control either the stator flux amplitude or the

Figure 3. Inverter voltage vectors and corresponding stator flux variation

torque. The vector ∆ ϕ S ≅ TC ⋅ U S can be decomposed in the component parallel and perpendicular to the stator flux; the parallel component modifies the stator flux amplitude while the perpendicular component controls the torque. SVM techniques have several advantages such as, lower torque ripple, lower Total Harmonic Distortion (THD) in the AC motor current, lower switching losses, and easier to implement in the digital systems. At each cycle period, this SVM

Table 1 summarizes the flux and torque change for applying the voltage vectors. The flux can be increased by the V1, V2, V6 vectors, whereas it can be decreased by V3, V4, V5 vectors. Similarly, torque is increased by the V2, V3, V4 vectors, but decreased by the V1, V5, V6 vectors. The zero vectors (V0 or V7) short-circuit the machine terminals and keep the flux and torque unaltered. Due to finite resistance drop, the torque and flux will slightly decreases during the short-circuit condition. 6

RECENT, Vol. 14, no. 1(37), March, 2013 Table 1. Flux and torque variations due to applied voltage vector (Arrow indicates magnitude and direction)

Voltage Vector

V1

V2

V3

V4

V5

V6

3. Fuzzy Duty Ratio Controller DTC Figure 4 shows a block diagram of the duty ratio-based DTC. In this control the selected inverter, switching state is applied for a portion of switching period and the zero switching state is applied for the rest of the period. During the zero switching state, the inverter applies zero voltage to the machine there by keeping the current and therefore the torque almost constant. The average input voltage to the motor during each switching state is then given by [13].

V7 (or) V0

ψs

0

Te The command stator flux and torque values are compared with the actual values in hysteresis flux and torque controllers, respectively. The flux controller is a 2-level while the torque controller is 3-level comparator. The digitized output signals of the flux (dψ) and torque (dm) controllers are as follows:

V = δ ⋅ V DC .

(5)

dψ = 1 for Eψ > +Hψ dψ = -1 for Eψ < –Hψ dm = 1, for E Te > +Hm dm = –1, for E Te < –Hm dm = 0, for –Hm